"They looked like they were having a fantastic time," says one observer of the Saturday night dinner. A source close to John adds, "They were hanging out this weekend."

Aniston, 39, in New York on business, dined with Mayer, 30, on oysters, ravioli with mushrooms and black truffles and fish.

The night before, Aniston and Mayer joined four others for a dinner at Mantra restaurant in Boston.

"They were friendly but regular," says a restaurant staffer. "We wouldn't have noticed them if someone hadn't called ahead. ... They just came in with their group and sat in the hookah bar and drank and ate."

The actress returned to Los Angeles on Monday.

Even though his summer romance with Aniston is now a thing of the past, Mayer, according to a friend, is "someone who is known for staying connected to people he cares about and for staying friends with people he's been linked to."

Not everyone remains convinced a re-match might be a good thing. Says one pal: "If they are back together, Jen is not bragging about it to her friends."